More pytests (#7)

* add constants.py

* more tests and robustness.

* try/catch JSONDecodedError.

tests/conftest.py

@@ -0,0 +1,8 @@
+import json
+import pytest
+
+
+@pytest.fixture
+def threads_api_response():
+    with open("tests/mock_data/threads_api_response.json") as json_file:
+        return json.load(json_file)